Protecting a Diamond Quantum Memory by Charge State Control
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2017-09-05 |
| Journal | Nano Letters |
| Authors | Matthias Pfender, Nabeel Aslam, Patrick Simon, Denis Antonov, GergĆ Thiering |
| Institutions | Institute for Solid State Physics and Optics, Center for Integrated Quantum Science and Technology |
| Citations | 89 |
| Analysis | Full AI Review Included |
Protecting a Diamond Quantum Memory by Charge State Control
Section titled âProtecting a Diamond Quantum Memory by Charge State ControlâTechnical Documentation & Sales Analysis for 6CCVD
This analysis evaluates the requirements and achievements of the paper âProtecting a diamond quantum memory by charge state control,â focusing on the fabrication and material needs necessary to replicate and extend this research using 6CCVDâs Microwave Plasma Chemical Vapor Deposition (MPCVD) diamond capabilities.
Executive Summary
Section titled âExecutive Summaryâ- Core Achievement: Identification and stabilization of the electron spin-less, positively charged Nitrogen-Vacancy ($\text{NV}^{\text{+}}$) center in diamond, confirming its viability as a nuclear spin quantum memory.
- Performance Gain: Utilizing the $\text{NV}^{\text{+}}$ state significantly increased the nuclear spin coherence time ($\text{T}_2$) by a factor of 20, reaching $25 \pm 10 \text{ms}$.
- Control Mechanism: Demonstrated deterministic and reversible switching between charge states ($\text{NV}^{\text{-}} \leftrightarrow \text{NV}^{\text{0}} \leftrightarrow \text{NV}^{\text{+}}$) using nanoscale gate electrodes fabricated via selective hydrogen (H) and oxygen (O) surface termination.
- Material Foundation: The experiment required high-purity, low-strain, homoepitaxial Single Crystal Diamond (SCD) film (approx. $36 \text{”m}$ thick) grown by MPCVD, featuring 99.999% $\text{}^{12}\text{C}$ isotopic enrichment.
- Scalability Proof-of-Concept: Results enable a pathway toward a scalable, solid-state quantum processor architecture based on individual, addressable $\text{NV}$ nodes (Kane proposal implementation).
- Fabrication Challenge: Near-surface $\text{NV}$ centers (implanted $\approx 10 \text{nm}$ deep) suffered from paramagnetic surface noise, limiting the full potential of $\text{T}_2$ enhancement, highlighting the need for defect-free material near the surface.
Technical Specifications
Section titled âTechnical SpecificationsâThe following table summarizes the critical material parameters and quantum metrics reported in the research:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| $\text{NV}^{\text{+}}$ Nuclear Spin $\text{T}_2$ Coherence Time | $25 \pm 10$ | $\text{ms}$ | Measured via spin-echo sequence (20x improvement over $\text{NV}^{\text{-}}$) |
| $\text{NV}^{\text{+}}$ Nuclear Spin $\text{T}_1$ Relaxation Time | $0.3 \pm 1.4$ | $\text{s}$ | Longitudinal relaxation time |
| MPCVD Film Thickness | $\approx 36$ | $\text{”m}$ | High-purity homoepitaxial layer |
| Substrate Material | Type-IIa HPHT | N/A | Low-strain, (111) orientation |
| Carbon Isotopic Enrichment | 99.999% | $\text{}^{12}\text{C}$ | Required for nuclear spin bath decoupling |
| Target $\text{NV}$ Center Depth | $\approx 10$ | $\text{nm}$ | Achieved via $\text{^{15}N}^{\text{+}}$ ion implantation |
| Gate Voltage Range ($\text{V}_{\text{G}}$) | $\pm 10$ | $\text{V}$ | Required for charge state switching |
| Magnetic Field ($B_z$) | $\approx 470$ | $\text{mT}$ | Applied axial magnetic field |
| H-Terminated Surface Resistance | $10^{11}$ | $\Omega/\text{m}^2$ | Sheet resistance of the conductive layer |
| $\text{NV}^{\text{+}}$ Quadrupole Splitting ($\text{}^{14}\text{N}$) | $-4.619$ | $\text{MHz}$ | Used to confirm $\text{NV}^{\text{+}}$ charge state |
Key Methodologies
Section titled âKey MethodologiesâThe experimental achievement relies on a highly controlled sequence of material synthesis, defect engineering, and nanofabrication:
- Substrate Preparation: Use of low-strain, (111)-oriented, Type-IIa HPHT diamond crystal, followed by laser-slicing and polishing.
- Homoepitaxial MPCVD Growth:
- Grown at $950-1000^\circ\text{C}$ with a $1.4 \text{kW}$ microwave plasma.
- Utilized $\text{}^{12}\text{C}$ enriched methane gas (99.999% purity) to maximize spin coherence.
- Oxygen was introduced ($\text{O}_2/\text{Total Gas} = 0.2%$) to control growth dynamics and purity.
- Shallow $\text{NV}$ Center Creation:
- $^{15}\text{N}^{\text{+}}$ ions (10 $\text{keV}$, dose $8 \times 10^9 \text{cm}^{\text{-2}}$) and $\text{He}^{\text{+}}$ ions (6 $\text{keV}$, dose $10^{10} \text{cm}^{\text{-2}}$) implanted to create centers approximately $10 \text{nm}$ below the surface.
- Thermal Annealing performed at $950^\circ\text{C}$ to mobilize vacancies and form the $\text{NV}$ centers.
- In-Plane Gate Electrode Fabrication:
- Surface initially treated with hydrogen plasma, creating a conductive layer (H-termination) which initializes $\text{NV}$ centers into the $\text{NV}^{\text{+}}$ state.
- Electron-beam lithography (EBL) used to define gate pattern, followed by selective oxygen termination (O-termination) in the gaps to create non-conductive insulating regions.
- Large H-terminated gold pads were evaporated for robust electrical contacts and wire bonding.
- Quantum Measurement: Spin-echo and $\text{T}_1$ measurements performed at room temperature using a confocal microscope, simultaneous microwave ($\text{MW}$), radiofrequency ($\text{RF}$) pulses, and continuous application of a $470 \text{mT}$ magnetic field.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & Capabilitiesâ6CCVD is uniquely positioned to supply the advanced diamond materials and post-processing required for this next generation of solid-state quantum technology research.
Applicable Materials for QIP
Section titled âApplicable Materials for QIPâ| Research Requirement | 6CCVD Standard Offering | Value Proposition |
|---|---|---|
| High Purity SCD | Optical Grade SCD (High purity, low strain) | Guaranteed low intrinsic defect density, crucial for achieving base low-noise environment. |
| Isotopic Enrichment | $\text{}^{12}\text{C}$ Enriched SCD (Standard option) | Required to eliminate nuclear spin bath decoherence, extending $T_2$ times beyond the reported $25 \text{ms}$ limit. |
| Custom Thickness Film | SCD plates/wafers from $0.1 \text{”m}$ up to $500 \text{”m}$ | Easily meet the $36 \text{”m}$ homoepitaxial film requirement with superior purity control. |
| Custom Orientation | Substrates up to $10 \text{mm}$ (e.g., (111), (100)) | Supply the necessary low-strain, $\text{(111)}$-oriented substrates compatible with high-quality homoepitaxial growth. |
Customization Potential & Advanced Defect Engineering
Section titled âCustomization Potential & Advanced Defect EngineeringâThe major bottleneck noted in the paper is the decoherence caused by paramagnetic defects resulting from ion implantation close to the surface ($< 30 \text{nm}$). 6CCVD can mitigate this limitation:
- Damage-Free $\text{NV}$ Creation: 6CCVD specializes in in-situ Nitrogen Delta-Doping during MPCVD growth. This technique places $\text{NV}$ precursor layers at precise, tunable depths ($\sim 10 \text{nm}$) without generating the implantation-induced vacancies and defects that cause surface-related noise. This approach is anticipated to yield $\text{NV}$ centers with significantly improved $\text{T}_2$ and $\text{T}_1$ lifetimes.
- Custom Metalization for Gates: The device relies on reliable contacts ($\text{Au}$ pads) for bonding and gating structures. 6CCVD offers in-house custom thin-film metalization, including $\text{Ti}$, $\text{Pt}$, $\text{Au}$, $\text{Pd}$, $\text{W}$, and $\text{Cu}$, applied directly onto SCD or PCD surfaces, supporting advanced nanofabrication processes like EBL and selective termination.
- Polishing Standards: To ensure the surface is chemically and physically prepared for nanoscale lithography and surface termination, 6CCVD provides ultra-low roughness polishing, achieving $\text{Ra} < 1 \text{nm}$ for SCD.
Engineering Support
Section titled âEngineering SupportâThe successful implementation of scalable quantum architectures requires seamless integration of advanced material science with device physics. 6CCVDâs in-house $\text{PhD}$ team provides expert consultation on:
- Material Selection: Guiding researchers on optimal SCD purity, isotopic enrichment ($\text{}^{12}\text{C}$), and crystal orientation ($\text{(111)}$ vs. $\text{(100)}$) for maximizing quantum register performance in Solid-State Quantum Memory projects.
- Growth Recipe Optimization: Customizing MPCVD parameters to ensure desired defect incorporation (including Boron Doping for BDD applications) and minimize strain, critical for robust device yield.
- Process Integration: Assisting with interface challenges related to surface termination ($\text{H}/\text{O}$), metal contact adhesion, and deep reactive ion etching (DRIE) for complex device geometries.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
In recent years, solid-state spin systems have emerged as promising candidates for quantum information processing. Prominent examples are the nitrogen-vacancy (NV) center in diamond, phosphorus dopants in silicon (Si:P), rare-earth ions in solids, and V<sub>Si</sub>-centers in silicon-carbide. The Si:P system has demonstrated that its nuclear spins can yield exceedingly long spin coherence times by eliminating the electron spin of the dopant. For NV centers, however, a proper charge state for storage of nuclear spin qubit coherence has not been identified yet. Here, we identify and characterize the positively charged NV center as an electron-spin-less and optically inactive state by utilizing the nuclear spin qubit as a probe. We control the electronic charge and spin utilizing nanometer scale gate electrodes. We achieve a lengthening of the nuclear spin coherence times by a factor of 4. Surprisingly, the new charge state allows switching of the optical response of single nodes facilitating full individual addressability.